Sir Malcom Rifkind has resigned from the gaffe-prone Conservative Friends of Russia Group after it launched a vicious attack on the Labour MP Chris Bryant and ran a demeaning photo of him in his underpants.

The former foreign secretary said the "inappropriate" photo was the last straw that had prompted him to quit as the group's honorary president.

"Sir Malcolm has had concerns for a considerable time," Rifkind's spokesman said. "He has been unhappy at the direction the group was going in politically, and the way people were behaving. This was the final straw. He decided to step down."

In a now-deleted post, the group lambasted Bryant and accused him of "ineffectual" leadership of the all-party parliamentary group on Russia, which has been critical of Vladimir Putin and the Kremlin's human rights record. Bryant, a former Europe minister and a vocal critic of Putin, described the photo as "puerile" and a slur on his sexuality.

In a scathing tweet, the MP for Rhondda wrote: "I gather the Conservative Friends of Russia have covered themselves in homophobic glory. Clearly [they] would prefer a Putin patsy to run the all-party group on Russia. Did the embassy pay for them?"

The Conservative Friends of Russia group was launched in August. Its stated aim was to improve relations between Moscow and London. The group enjoyed the support of several prominent Conservatives, including Rifkind; John Whittingdale, who chairs the culture select committee; and the MPs Nigel Evans and Robert Buckland.

Conservative Friends of Russia insists it receives no money from the Russian state. Bryant, however, has repeatedly questioned the organisation's sources of funding.

He tweeted: "The point is your craven attitude to Russian abuses – and being funded by the Russian state."

On Friday Bryant welcomed Rifkind's decision to quit and called on Whittingdale and other Tory MPs linked to the organisation to resign from it.

In an email to the Guardian, Royal said: "We are of course extremely disappointed by Sir Malcolm's resignation and have thanked him for his help and support so far." Sources at the group said he had "got the wrong end of the stick".